The very interesting and recommended book The Silent Deep: The Royal Navy Submarine Service Since 1945 by Peter Hennessy and James Jinks gives the annual cost of maintaining the deterrent as 1.5% of the annual benefits bill.
Meanwhile, there was an article in The Times on Tuesday about the current detachment of B-1s, B-2s and B-52s at Fairford, and the comments section certainly showed that ignorance among the general public is alive and kicking.